The present invention relates generally to the art of electrochemical cells and more particularly autoclavable electrochemical cells or batteries such as may be used, for example, in implantable medical devices.
Numerous power sources have been developed for use in implantable devices such as implantable drug pumps and pacemakers. It is important that the medical devices be sterilized prior to implantation in the body. Medical devices have been sterilized by treatment with an oxide gas such as ethylene oxide (filling oxide gas treatment). However, in addition to being considered environmentally unsafe, ethylene oxide gas is necrotic to tissue. During sterilization the ethylene oxide gas may become trapped in spaces within a medical device with the result that its eventual release, after implantation of the device, may lead to potentially severe tissue damage in the patient.
An alternative to ethylene oxide gas treatment is sterilization of the medical device in an autoclave. For such sterilization the implantable medical device and the electrochemical cell which serves as its power source must be capable of withstanding the repeated prolonged exposures to heat soak and other autoclaving conditions at the high temperatures on the order of 130 to 135 degrees C. encountered.
Batteries for implantable medical devices may include anodes having as active material lithium or other alkali metal, cathodes having as active material silver vanadium oxide or other metal oxide or carbon monoflouride, electrolytes composed of a lithium salt and an organic solvent, and a, separator material between the electrodes and which is porous for passage of the electrolyte therethrough for ionic transfer between the electrodes for generating a current. Examples of such batteries are disclosed in U.S. Pat. Nos. 4,057,679; 4,618,548; and 4,830,940. While batteries have been provided which have operating temperatures within the range of minus 55 to plus 225 degrees C., as discussed in related U.S. Pat. Nos. 4,310,609 and 4,391,729, which are assigned to the assignee of the present invention, the ability of a battery to operate in such a temperature range does not determine whether it has the ability to withstand the heat soak and other conditions of autoclaving at temperatures of about 130 to 135 degrees C.
Other patents which may be of interest include U.S. Pat. Nos. 4,751,157; 4,751,158; 4,146,685; 4,574,113; 4,615,959; 4,668,594; 4,668,595; and 4,735,875.
As discussed by the inventors of the present invention in an article entitled xe2x80x9cAutoclavable Li/Silver Vanadium Oxide Cellxe2x80x9d, Progress in Batteries and Solar Cells, Volume 8 (1989), at pages 122-125, a desirable characteristic of some medical cells is the ability of the cells to withstand repeated high temperature excursions that occur during sterilization in an autoclave without loss of deliverable capacity.
Such batteries as disclosed in the aforesaid patents are deficient for purposes of autoclaving since their compositions are such that one or more of their components may render the cell dimensionally and/or chemically unstable during repeated exposures at autoclavable temperatures or cause a significant reduction in the cell""s capacity as a consequence of such exposures.
It is accordingly an object of the present invention to provide an electrochemical cell which can withstand repeated exposure to autoclave environments without significant loss of capacity.
In order to provide such an autoclavable electrochemical cell, in accordance with the present invention the anode is provided to have as active material a material which has a melting point greater than about 150 degrees C. and the solvent for the electrolyte is characterized by having a boiling point greater than about 100 degrees C. and a dielectric constant greater than about 5.